The present invention relates to television tuners and, in particular, to television tuners employing micro-electro-mechanically switched tuning matrices, which tuning matrices may include capacitance and/or inductance elements.
From the early days of radio, the need for tunable resonant electrical circuits was recognized. Large mechanical tuning elements, such as air-dielectric capacitors and air-core inductors, in time gave way to smaller, more efficient capacitors and inductors. In the continuing evolution from vacuum tubes to transistors to integrated circuits, the trend has been for ever-decreasing size and cost. To this end, micro-electronic circuits and integrated electronic circuits have become the mainstay of modern-day electronics.
In the field of television (TV) tuners and other superheterodyne receivers, for example, this evolution has seen the vacuum tubes and multi-gang mechanical switches with discreet capacitors, inductors and resistors soldered thereon yield to transistorized printed-wiring circuit boards, and the transistorized circuit boards yield to micro-electronic and integrated circuits mounted on printed-wiring substrates. But even modem integrated circuit TV tuners still employ discrete components for the capacitive and inductive tuning elements.
The electronically-controllable variable tuning elements currently employed are semiconductor varactor diodes which exhibit a capacitance that varies inversely to the magnitude of the DC reverse bias voltage applied thereto. Varactor diodes are coupled to inductors or to a transmission line having inductive reactance to form resonators that are employed in the pre-selector filters, post-selector filters and oscillators of tunable receivers such as modern TV tuners.
For example, FIG. 1 shows a conventional tunable circuit of this sort in which the resonant frequency is determined by the value of the capacitance exhibited by varactor diode D2 and the inductance of inductors L01 and L02. PIN diode D1 provides band switching under the control of voltage VD1. With switching voltage VD1 at +20 volts, diode D1 is open (nonconductive) and inductors L01 and L02 in series form the inductance of the tunable circuit; and with switching voltage VD1 at xe2x88x9220 volts, D1 is conductive substantially shorting inductor L01, thereby leaving L02 as the inductance of the tunable circuit. Varactor diode D2 exhibits a variable capacitance in response to tuning voltage VD2 changing between about +1 to +20 volts. Capacitors CD1 and CD2 are needed to provide DC isolation for the control voltage VD1 and the tuning voltage VD2, respectively, and have capacitances sufficiently large as not to undesirably affect the resonant frequency of the tunable circuit. Thus, the need for discrete electronic components and for additional components for DC isolation tends to increase the size, assembly difficulty and the cost of these products, all of which are not desirable.
Unfortunately, varactor diodes also have undesirable electrical characteristics that limit their usefulness and the performance obtainable. Firstly, the capacitance of a varactor diode is a non-linear function of its reverse bias voltage, thereby being a source of distortion of the signals applied to or passed through the varactor diode. Secondly, varactor diodes are relatively lossy and so will exhibit a relatively low Q. The effect of a low Q on the tuned circuits of a typical TV tuner is to produce greater signal losses, to limit the sharpness, selectivity and narrow bandwidth capability of filters, and to increase the overall noise figure, and thereby increase the signal-to-noise ratio, of the tuner.
Accordingly, there is a need for tunable circuits that will have lower distortion, higher Q, and improved filter characteristics, and that will enable tuners having lower distortion, improved image rejection and adjacent channel rejection, and a lower noise figure.
To this end, the tuner of present invention comprises a tunable bandpass filter on a substrate having a passband including a resonant frequency responsive to a tuning control signal, a tunable oscillator on the substrate generating a controllable frequency signal responsive to a frequency control signal, and a mixer on the substrate coupled to the tunable bandpass filter for receiving signals in the passband and coupled to the tunable oscillator for receiving the controllable frequency signal. The tunable bandpass filter includes a resonant circuit comprising a plurality of capacitors formed of conductive layers and dielectric layers on the substrate; and a plurality of switches formed of layers of materials on the substrate, wherein the switches are selectively opened and closed by movement of a switch arm in response to the tuning control signal, and wherein ones of the plurality of switches selectively couple respective ones of the plurality of capacitors to a conductive connection on the substrate. A tuning control generates the tuning control signal and the frequency control signal.
The present invention also comprises a method for fabricating a matrix of a plurality of capacitors and electro-mechanical switches connected in circuit on a substrate by:
depositing a conductive layer on parts of the substrate to form a plurality of capacitor plates, and to form a plurality of switch contacts and a plurality of control conductors associated with respective ones of the plurality of switch contacts;
depositing a dielectric layer on each of the plurality of capacitor plates and another conductive layer on each dielectric layer to form the plurality of capacitors on the substrate;
forming a removable layer overlaying the plurality of switch contacts and at least portions of the plurality of control conductors associated therewith, the removable layer having a plurality of holes therethrough with one of the holes proximate to each control conductor;
depositing a second conductive layer on the removable layer, the second conductive layer forming a plurality of conductive areas, each conductive area overlying a respective one of the control conductors, a respective one of the switch contacts and being attached to the substrate through a respective one of the holes to form a respective switch arm associated with one of the plurality of switch contacts;
removing the removable layer to leave the plurality of switch arms spaced apart from the substrate and attached thereto at one end thereof and spaced apart from the respective switch contact associated therewith at its other end; and
depositing a plurality of conductive connections between ones of the capacitors, ones of the switch arms and ones of the switch contacts to connect ones of the capacitors and the electro-mechanical switches in circuit on the substrate.